The invention relates to a circuit arrangement for gaining a DC output voltage from a DC input voltage. The DC output voltage should be offset-compensated, i.e. in the case where a DC output voltage terminal is coupled to another potential, the potential of the other DC output voltage terminal occurs in a corresponding offset-compensated form, i.e. taking the potential jump of the other terminal into account.
Such circuit arrangements are desirable, for example, for bus systems, particularly in vehicles. In bus systems in which the power supply of the bus participants takes place via the bus lines, a power supply for the participants should also be ensured when one of the bus lines is coupled to a fixed potential, for example, vehicle chassis.
A circuit concept is known from EP 0 858 174 A2 in which this problem is solved by means of a transformer. For many applications, particularly in vehicles, the use of transformers is, however, not very popular. Transformers are expensive and sensitive so that they may break, particularly in vehicles. Their high own weight is critical at fast accelerations as may occur, for example, in the case of accidents. It is exactly in these cases that a flawless functioning of the power supply is of particular importance.
It is an object of the invention to provide a circuit arrangement of the type described in the opening paragraph which can supply an offset-compensated DC output voltage and has a possibly simple structure without using transformers.
This object is solved by the following characteristic features defined in claim 1.
A circuit arrangement for gaining an offset-compensated DC output voltage from a DC input voltage, wherein means for generating an alternating voltage from the DC input voltage are provided, and wherein the alternating voltage is applied to at least one rectifier stage which comprises two coupling capacitors and a subsequent full-bridge rectifier at its input and is coupled at its output to at least an output capacitance via which the DC output voltage decreases.
In this concept, an alternating voltage is first gained from a DC input voltage. The alternating voltage is then applied to a rectifier stage which comprises two coupling capacitors at its input. The coupling capacitors precede a full-bridge rectifier which generates a direct voltage from the alternating voltage again. At its output, the circuit arrangement comprises an output capacitance to which the two terminals of the full-bridge rectifiers are coupled. Thus, a direct voltage decreases across the output capacitance, which, inter alia, has the advantage that, as regards its potential, it is separated from the potential of the DC input voltage, in any case for direct voltage and alternating voltages of lower frequencies.
When a short circuit occurs at one of the two terminals of the output capacitance, thus, for example, at one of the two lines of a subsequent bus system, the two coupling capacitors generate the required offset for the DC output voltage. When such a short circuit occurs, output currents initially flow in the capacitors until the DC component of the capacitor voltages has adjusted to the required offset.
In this way, an offset-compensated DC output voltage is obtained which is effective against short circuits of a terminal of the DC output voltage, against another direct voltage, or, dependent on the dimensioning of the coupling capacitors, also against a low-frequency alternating voltage.
A plurality of rectifier stages may be provided so that a DC output voltage which is larger than the DC input voltage can be generated.
The circuit arrangement according to the invention achieves the described objects without any transformer and by means of a relatively simple, purely electronic circuit. It is therefore relatively economical and particularly does not comprise heavy components which, when used in, for example vehicles, could break in the case of high accelerations.
The circuit arrangement may be specifically used in other applications for generating a positive, negative or positive and negative output voltage in that a desired potential is adjusted by a corresponding coupling of a terminal of the DC output voltage.
The circuit arrangement may be used, for example, also for battery-operated apparatuses in which a minimal number of cells should be used or in which given, also higher DC voltages should be generated with the desired polarity in the apparatus.
An embodiment of the invention as defined in claim 2 has the object to provide a DC output voltage which is higher as compared with the DC input voltage. A plurality of rectifier stages with two coupling capacitors each is provided, to which the alternating voltage is applied. The outputs of the rectifier stages are serially arranged one behind the other for the purpose of voltage multiplication. At least one output capacitance is arranged via the series arrangement of the outputs of the rectifier stages so that the sum of the direct voltages of the individual rectifier stages decreases at the output capacitance and is available as a DC output voltage.
As defined in claim 3, it is also possible to provide one output capacitance for each rectifier stage. The output voltage of the circuit arrangement then decreases across the series arrangement of the output capacitances.
In accordance with a further embodiment of the invention as defined in claim 5, a H bridge is advantageously provided for generating the alternating voltage, which H bridge generates an alternating voltage from the DC input voltage by using simple electronic switches and a corresponding switching voltage.
As defined in claim 6, the circuit arrangement according to the invention can be advantageously used for generating a DC power supply voltage for a data bus. The advantages described above particularly apply to the use in a vehicle, as defined in claim 7. Low costs, high reliability and disturbance security, also in the case of accidents, are important aspects. When the circuit arrangement according to the invention is used in a data bus in such a vehicle, a power supply of the bus participants via the data bus can also be ensured when one of the two bus lines, which may be simultaneously power supply lines, is coupled to a potential, for example, vehicle chassis.
As defined in claim 8, the circuit arrangement according to the invention is, however, also suitable for generating desired direct voltages in a relatively simple way in battery-operated apparatuses without the battery having to supply these voltages as regards quantity and sign.